The formation of deciduum following implantation is considered to function as a protective barrier to the embryo from the maternal insults. Thus, defective implantation and uterine decidualization may account for early pregnancy losses. Identification of embryonic cell-type(s) and embryonic factor(s) for implantation and decidualization will be a major achievement to improve fecundity. The tight junction (TJ) that creates a barrier restricting the paracellular permeability is present only in cells of the epithelium, but not in the stroma. We noticed for the first time that the stromal cells of the primary decidual zone (PDZ) surrounding the implanting blastocyst, but not the secondary decidual zone (SDZ), on day 6 of pregnancy express the TJ and its associated proteins, occludin, claudin-1, and zonula occludens-1 (ZO-1) and ZO-2. Our preliminary results show that the trophoblast (Tr), but not the inner cell mass (ICM), of the blastocyst induces implantation and decidualization reactions with correct expression of ZO-1 in the PDZ. We also provide evidence here that heparin binding-epidermal growth factor-like growth factor (HB-EGF) produced by the blastocyst is important for implantation, decidualization, and ZO-1 expression in the PDZ. These observations together with the restriction of passage of macromolecular dye to the PDZ induced by normal blastocysts after intravenous administration, but not by artificially induced decidualization, suggest that the embryonic HB-EGF influences implantation, the PDZ formation, and genesis of the TJ in the PDZ which creates an avascular permeability barrier to protect the embryo. Therefore, our specific aims are to determine in a murine model: 1) Influence of the Tr in implantation and the PDZ formation, 2) Influence of embryonic HB-EGF in implantation and the PDZ formation, 3) Influence of HB-EGF in the transformation of stromal cells to decidual cells with epithelial characteristics, and 4) The importance of the PDZ as a barrier at the maternal-fetal interface. We will use multiple experimental approaches including RT-PCR, Northern and in situ hybridizations, immunohistochemistry/ immunofluorescence, Western blotting, phosphorylation, electron microscopy, embryo culture and transfer, cell culture, in vivo/in vitro adenoviral vector-driven gene delivery, and others to accomplish our goals. These results will further our understanding regarding uterine protective mechanism(s) to the safeguard of the implanting embryo during early pregnancy.